Originally posted by Cody
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To shed some light on your observations - pressure and flow are both dependent on the resistance provided by the puck. Less resistance = more flow and a lower pressure. The resistance is dependent not only on grind size, but also on the viscosity (which depends on what/how much is extracted from the coffee) and probably also how much carbon dioxide there is.
As the coffee ages, there are various chemical reactions occurring which impact the composition / concentration of the extracted material and thus the viscosity. There is also an off-gassing of CO2, which is why you get less crema.
As the others have said, fresher is better (to a point).

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